Growth hormone (GH), insulin-like growth factor (IGF-I), and prolactin (PRL) can influence various aspects of reproductive functions in both females and males. However, the physiological role of PRL and the GH-IGF-I axis in the control of reproduction has been difficult to define, and the recent availability of knock-out (KO) animals allows re-examination of this issue. PRL-receptor (R)-KO and PRL-KO females are sterile because of luteal failure. In addition, these mice have severe deficits in the development of oocytes and early embryos. However, male fertility is not affected in the PRL-KO and in most of the PRL-R-KO animals. IGF-KO animals have an infantile reproductive system and are sterile. GH-R-KO mice can reproduce, but their breeding performance is reduced, particularly in females. These data indicate that IGF-I signaling is required for normal reproductive development and confirm the requirement for PRL for fertility in the female mouse. GH resistance leads to quantitative deficits in reproductive development and functions, but does not preclude fertility in either sex. We suspect that PRL and the GH-IGF-I axis provide partially overlapping (redundant) regulatory inputs to the hypothalamic-pituitary-gonadal axis, and consequently, targeted disruption of either signaling pathway has relatively mild consequences on many functions related to reproduction. Overexpression of heterologous or homologous GH in transgenic animals can lead to severe reproductive deficits, including female sterility in some of the lines. Studies in GH transgenics should allow the identification of mechanisms that mediate the effects of chronic overexposure to GH on reproduction.